1. Field of the Invention
The present invention relates to a bump forming method for forming a bump on the electrode of a workpiece.
2. Description of the Related Art
A method using the wire bonding technique is known as a method for forming a bump (protruded electrode) on the electrode of a workpiece such as an IC chip. A conventional bump forming method is described below.
FIGS. 19A, 19B, 19C, and 19D, and FIG. 20 are illustrations showing a conventional bump forming method. As shown in FIGS. 19A, a ball 4 is formed at the bottom end of a wire 2 extended from a capillary tool 1 by making a torch 3 approach to the bottom end of the wire 2 and generating spark therebetween. Symbol 5 denotes a first clamper located above the capillary tool 1 to hold the wire 2, and 6 denotes a second clamper located above the first clamper 5 to apply a tension to the wire 2.
Then, as shown in FIG. 19B, the ball 4 is pressed against the electrode of a workpiece 7 by lowering the capillary tool 1. Symbol 8 denotes a base on which the workpiece 7 is mounted. Then, as shown in FIG. 19C, the wire 2 (also referred to as tail) is conducted and extended from the bottom end of the capillary tool 1 by a length necessary to form the next ball by slightly raising the capillary tool 1 and thereafter, the wire 2 is securely held by the first clamper 5. Then, as shown in FIG. 19D, the first clamper 5 and the capillary tool 1 are raised while holding the wire 2 to thereby cut the wire 2 immediately above the ball 4 and thus, a series of operations are completed and the bump 4 is formed on the electrode of the workpiece 7.
FIG. 20 shows an enlarged state in which the ball 4 is bonded to the electrode of the workpiece 7 and then the wire 2 is cut off while holding the wire 2 by the first clamper 5 and raising the wire 2 in the step shown in FIG. 19C. Symbol "A" denotes a cutting line and la denotes the central hole of the capillary tool 1 through which the wire 2 is inserted. By raising the wire 2, the wire 2 is forcibly cut at the cutting line "a" immediately above the ball 4. In this case, a large tensile stress is produced on the joint surface between the ball (bump) 4 and the electrode 7a of the workpiece 7. Therefore, there are problems that the ball (bump) 4 is easily removed from the electrode 7a and the joint surface between the ball (bump) 4 and the electrode 7a is easily damaged. In FIG. 20, a black portion at the bottom of the ball 4 is a damaged portion.